U.S. Pat. No. 5,253,156 corresponding to JP-A-8-34709 discloses a semiconductor integrated circuit including a main cell having a lateral insulated gate bipolar transistor (IGBT) and a current detector (hereafter referred to as “sense cell”) for detecting an electric current flowing through an emitter of the IGBT. The sense cell has a lateral IGBT with the same structure as the lateral IGBT of the main cell except the length of the emitter. The IGBTs are coupled in a current mirror configuration so that an electric current flowing through the emitter of the IGBT of the sense cell becomes smaller than the electric current flowing through the emitter of the IGBT of the main cell by a predetermined ratio that depends on the ratio between the lengths of the emitters. Thus, the current flowing through the main cell can be detected based on the current flowing through the sense cell.
However, when the semiconductor integrated circuit disclosed in U.S. Pat. No. 5,253,156 is used to form a circuit shown in FIG. 14, the following disadvantages may occur.
In the circuit shown in FIG. 14, an electric current flowing through a main cell 1000 is detected based on an electric current flowing through a sense cell 1001, and the electric current flowing through the sense cell 1001 is detected based on a voltage across a resistor Rs. When a resistance of the resistor Rs is increased to increase the voltage, an emitter potential of the sense cell 1001 is increased. Accordingly, a potential of a p-type body layer, which is electrically connected to an emitter electrode, is increased. As a result, a PN junction between the p-type body layer and an n−-type drift layer is forward biased, an output (i.e., the voltage across the resistor Rs) becomes unstable. In order to stabilize the output voltage, it is preferable that the output voltage should be limited up to 0.3 volts (V). Further, when a high voltage (e.g., from 200V to 600V) is applied to a collector, the output voltage can have an error due to a coupling with the high voltage.
Such a problem can occur in a semiconductor element other than an IGBT.
By the way, such a circuit shown in FIG. 14 is used to form an inverter circuit. For example, US 2009/0057832 corresponding to JP-A-2009-268054 discloses a method for detecting a current in the inverter circuit by using a shunt resistor. The method disclosed in US 2008/0246426 is directed to detect a value of the current, not a direction of the current. It is preferable to detect not only the value of the current, but also the direction of the current in order to perform sensorless sinusoidal control of the current as disclosed in US 2008/0246426 corresponding to JP 4396762.